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Séminaires
Test
The Centre for Systems Engineering and Applied Mechanics (CESAME) organizes a series of seminars. The seminars are held in the CESAME lecture room, Building EULER, 4-6, av. Georges Lemaître, Louvain-la-Neuve (Parking 13).
For further information, feel free to contact the secretary: 010/47.80.36.
If you wish to receive the seminar announcements by email, please send an email to Lydia De Boeck.
Look also at the seminars of the departement of mecanics and at the
seminars of the research group on Large Graphs and Networks.
Seminars to come
23/03/2010 (14.00) [Lieu : Bât. Euler] Olivier
BONAVENTURE (UCL-INGI) "One path is not enough anymore"
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The Internet was designed with the assumption that there was one path
between a pair of communicating hosts. The TCP congestion control scheme
was added in the late 1980s to avoid congestion collapse on bottleneck
links. The growth of multihoming, both at the enterprise/campus network
level and at the host level with multiple wireless interfaces,
completely changes the situation since there are now several useable
paths between communicating hosts. In this talk, I'll survey the broad
implications of this change on the network, transport and application
layers of the Future Internet.
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6/04/2010 (14.00) [Lieu : Bât. Euler] Samih
ZEIN "Polynomial Chaos Expansion in Reliability Analysis and Robust Optimisation"
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The Polynomial Chaos Expansion (PCE) is an efficient numerical method for relating
the output of a nonlinear system with the uncertainty in its input parameters.
The output can be approximated with a multidimensional polynomial function using
the so called PCE. Numerically, such approximation can be obtained using a linear
regression with an adequate design of experiments. In this presentation, a review
is given on the PCE principle and its application in reliability analysis and robust
optimisation. Then, an optimization algorithm is proposed that seeks the best design
of experiments in the D-optimal sens for the PCE. This algorithm is based on the heuristic
optimisation and the Fedorov exchange algorithm and is adapted to the specific case of the
polynomial chaos expansion. We will conclude the presentation with a brief overview of the
integration of a PCE method into an industrial optimization platform and task manager."
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13/04/2010 (14.00) [Lieu : Bât. Euler] Emilie
MARCHANDISE (CESAME) "Quality meshing of medical geometries with harmonic map"
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This work describes an automatic approach to recover a high quality surface mesh from low-quality
or oversampled inputs (STL-files) obtained from medical imaging through classical segmentation
techniques. The approach combines a robust method of parametrization based on harmonic maps
with a recursive call to a multi-level edge partitioning software. By doing so, we are able to get rid
of the topological and geometrical limitations of harmonic maps. The overall remeshing procedure
is implemented, together with the ïnfinite element discretization procedure required for computing
harmonic maps, in the open-source mesh generator Gmsh. We show that the proposed method
produces high quality meshes and we highlight the benefits of using those high quality meshes for
biomedical simulations.
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20/04/2010 (14.00) [Lieu : Bât. Euler] Guillaume
LECLERCQ (CESAME) "The brain accounts for 3D eye and head kinematics in the visuomotor transformation of velocity signals in manual tracking"
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Visually guided arm movements are common actions carried out each day in our everyday life. When performing visually guided arm movements,
the brain needs to transform the visual information (the tennis ball, an obstacle) into an appropriate motor plan for the arm (i.e. a command to give
to the arms muscles). Here, we investigated if the brain accounts for the 3D eye position, 3D head position and 3D eye velocity in the visuomotor
transformation of velocity signals for manual tracking movements. A visuomotor transformation model taking into account the complete 3D eye-head-shoulder
kinematics was built using dual quaternions. To test model predictions, human subjects performed manual tracking movements in different eye-head-shoulder
configurations, either static or dynamic. We compared the arm initial open-loop direction with the predictions provided by the model. Results show that the brain
uses internal 3D eye and head position as well as eye velocity signals to account for the eye-head-shoulder kinematics in the visuomotor transformation of velocity
signals for manual tracking movements.
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27/04/2010 (14.00) [Lieu : Bât. Euler] Didier
HENRION (LAAS-CNRS, University of Toulouse
) "Occupation measures and LMI formulation of
piecewise affine optimal control design problems"
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After recalling the mathematical framework of
occupation measures for nonlinear dynamical systems control,
we show how linear matrix inequalities (LMI) can be used
to solve computationally the corresponding generalized
problem of moments. We illustrate these ideas first
on a simple linear quadratic regulator design problem,
and then on a less trivial piecewise affine optimal control
problem for which we know analytically the solution
of the Hamilton-Jacobi-Bellman equation. This is joint work
with Luis Rodrigues, Concordia University, Montreal, Canada.
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Previous seminars
02/03/2010 (14.00) [Lieu : Bât. Euler] Giovanni
SAMAEY
(CESAME) Numerical properties of equation-free methods for multiscale problems
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In quite a number of problems, there exists a wide gap between the (microscopic) level at which a system is modeled and the (macroscopic)
space and time scales at which one would like to observe the system. Equation-free methods try to bridge this gap by constructing a coarse time-stepper
for the macroscopic observables as a three-step procedure: (1) lift (from macro to micro); (2) evolve (using the microscopic model); and (3) restrict (from micro to macro).
We will discuss both how to construct lifting operators to obtain a well-defined coarse time-stepper, as well as what macroscopic tasks can be performed once a coarse time-stepper is available.
Particular emphasis will be put on the numerical properties of the resulting methods (accuracy and efficiency) and on the use of approximate macroscopic models to accelerate the computations.
Collaborators that have contributed to this work will be acknowledged throughout the talk.
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23/02/2010 (14.00) [Lieu : Bât. Euler] Manuel
MARTINEZ (University of Seville
) "Stability of Asynchronous Feedback-Interconnected Dissipative Systems"
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This work presents a method to analyze the stability of the
feedback interconnection of a class of dissipative linear systems
when the sampling associated to the feedback interconnection is
asynchronous. We consider systems that are either Input/Output
Strictly passive (IOSP), or systems which have bounded L2-gains
less than one. The analysis is performed by using the concept of
MAximum Sampling time preserving Dissipation (MASD), for each
interconnected system. We investigate the impact of using the
scattering transformation in the computation of the MASD, and we
provide a numerical algorithm (based on a set of LMI's) that allows
to choose the most suitable configuration for the interconnection.
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18/02/2010 (16.30) [Lieu : Auditoire Ch.-J. de La Vallée Poussin (cycl 01) ] Jean-Charles
DELVENNE (FUNDP Namur) "Colloquium CESAME-MAPA "Random walks on graphs: what for?"
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How does Google work? What is so magical about expander graph? How do we
detect communities in large social graphs? The three answers involve one
same tool: random walks on graphs.
Given a graph, one may start from a vertex and, at each time step, jump
randomly to one of its neighbours. This random walk transforms the graph
into a Markov chain. The study of this Markov chain provides a lot of
insight into the combinatorial properties of the graph. We will cover a
sample of applications of this technique.
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11/02/2010 (16.30) [Lieu : Auditoire Euler] Pierre
BIELIAVSKY (UCL) "Colloquium CESAME-MAPA "Géométrie non commutative et théorie de Lie."
(slides)
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Mon but est d'exposer de manière élémentaire les concepts à la base de
la géométrie non commutative d'Alain Connes et, si le temps le permet,
de montrer certaines implications en géométrie différentielle et en
théorie de Lie.
L'ensemble C(X) des fonctions continues à valeurs complexes sur un
espace topologique X a naturellement une structure d'algèbre
associative commutative. En effet, étant données deux fonctions
f:X-->C et g:X-->C, leur produit est la fonction f.g définie par
(f.g)(x)=f(x).g(x).
Dans de nombreux cas, cette structure d'algèbre encode complètement la
structure topologique de l'espace X. Ce résultat classique,
dû à I. Guelfan'd, fournit un outil algébrique pour l'étude des
espaces topologiques.
Dans les années 80, Alain Connes introduit la géométrie différentielle
non commutative basée sur l'idée que des situations géométriques
apparemment disparates, incluant la géométrie riemannienne et la
théorie des feuilletages, peuvent elles aussi être encodées par une
notion algébrique commune: celle de triplet spectral.
Dans mon exposé, j'introduirai les paradigmes de bases de cette
théorie en me basant sur un concept émergeant de la théorie quantique:
la notion de déformation.
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09/02/2010 (14.00) [Lieu : Bât. Euler] Mariya
ISHTEVA (CESAME) "Numerical methods for the best low
multilinear rank approximation of higher-order tensors".
(slides)
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Higher-order tensors are generalizations of vectors and matrices to third-
or even higher-order arrays of numbers. The multilinear rank of a tensor
is a direct generalization of column and row rank of a matrix. In this
talk we discuss the best low multilinear rank approximation. Given a
higher-order tensor, we are looking for another tensor, as close as
possible to the original one and with multilinear rank bounded by
prespecified numbers. This approximation is used for dimensionality
reduction and signal subspace estimation in higher-order statistics,
biomedical signal processing, telecommunications and many other fields. We
consider two particular applications, present several algorithms for the
computation of the approximation and discuss the issue of local minima.
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16/12/2009 (16.15) [Lieu : Auditoire Ch.-J. de La Vallée Poussin (cycl 01)
] Marc
BURGER (ULg) Colloquium CESAME-MAPA
"Surfaces, espaces de Teichmüller et groupes de Lie"
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Il est bien connu depuis Nielsen que les surfaces (compactes connexes sans bord) sont classifiées, à homéomorphisme près, par leur groupe fondamental.
Cette situation change radicalement lorsqu'on adopte le point de vue de la géométrie différentielle: une surface S donnée porte en effet de nombreuses structures Riemanniennes différentes.
En fait, à toute métrique hyperbolique sur S on associe une représentation linéaire de son groupe fondamental $\pi_1(S)$ à valeurs dans SL(2,R).
On obtient ainsi un ensemble de représentations, appelé "espace de Teichmüller", qui porte une structure très riche. L'objet de cet exposé est de décrire
ce qui se produit lorsqu'on remplace SL(2,R) par un groupe de Lie de dimension supérieure.
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15/12/2009 (14.00) [Lieu : Bât. Euler] Quentin
RENTMEESTERS (CESAME) "Filtering on manifolds"
(slides)
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In many applications related to signal processing and image processing, a
filtering technique is required to reduce the influence of perturbations
on the measurements. In this talk, we will see different approaches to
implement such a filtering technique when the measurements belong to a
nonlinear space. We will illustrate these techniques on the sphere and
show some concrete applications related to the Grassmann manifold.
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08/12/2009 (14.00) [Lieu : Bât. Euler] Charlotte
BEAUTHIER (FUNDP) "Modelling Competitive Coexistence in a Chemostat by LQ Type State Feedback"
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The chemostat model describes the interaction of microbial species which
are competing for a single nutrient. It has been used for different systems such
as lakes, waste-water treatment processes and biological reactors producing ge-
netically altered organisms. The competitive exclusion principle is a theoretical
result which states that the competition process yields at best a single winning
species in the long run. However, in nature, many species seem to coexist. This
contradiction between the theory and the real world is our main motivation for
modifying the model in order to try to bring them in better accordance.
In this talk, the problem is stated in terms of an LQ-optimal control problem
for a linearized system in order to garantee the coexistence of species for the
(nonlinear) chemostat model. Conditions are reported such that the computed
linear state feedback can be applied to the nonlinear system and garantee the
well-posedness of the model and in particular the coexistence of the species.
The results are illustrated by some numerical experiments.
This contribution is a joint work in progress with Joseph Winkin (FUNDP)
and Denis Dochain (UCL).
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01/12/2009 (14.00) [Lieu : Bât. Euler] Damien
ERNST (ULg) " Learning about near-optimal policies from a sample of trajectories"
(slides)
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In many areas of the medical, social and engineering
sciences the following planning problem arises. A training set of
trajectories representing the system state, the actions and the
instantaneous rewards at successive discrete instants is known and a
decision policy leading to high cumulative rewards has to be found. In
the first part of this talk, I will present some batch mode
reinforcement learning algorithms for solving these types of problems.
Afterwards, I will report on some recent work to build lower bounds on
the performance of a policy from trajectories. Finally, I will show
that this work has opened a frontier in batch mode reinforcement
learning whose final implications we cannot yet guess.
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24/11/2009 (14.00) [Lieu : Bât. Euler] Luc
VANDENDORPE (ELEC/TELE, UCL) "Rate-Optimized Power Allocation for Relayed OFDM transmission in different scenarii"
(slides)
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We consider an OFDM (orthogonal frequency division multiplexing) point-to-point transmission scheme which is enhanced by
means of a relay. Symbols sent by the source during a first time slot may be (but are not necessarily) retransmitted by the relay
during a second time slot. The relay is assumed to be of the DF (decode-and-forward) type. For each relayed carrier, the destination
implements maximum ratio combining. Two protocols are considered. Assuming perfect CSI (channel state information), the
paper investigates the power allocation problem so as to maximize the rate offered by the scheme for two types of power constraints.
Both cases of sum power constraint and individual power constraints at the source and at the relay are addressed. The theoretical
analysis is illustrated through numerical results for the two protocols and both types of constraints.
Then the case where multiple relays are active will be considered. Finally a multiple access scheme with 2 users will be discussed.
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17/11/2009 (14.00) [Lieu : Bât. Euler] Eric
BULLINGER (ULg) "A Systems Biology Approach to Apoptosis Signalling"
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| Apoptosis is an important physiological process crucially involved in
the development and homeostasis of multi-cellular organisms. Although
the major signalling pathways leading from the extrinsic induction to
the execution of apoptosis have been unravelled, a detailed mechanistic
understanding of the complex underlying network and the signal crosstalk
remains elusive. A systems biology approach allows to combine diverse
data into mathematical models to perform predictive simulations and
testing of quantitative and dynamical hypotheses. The modelling process
furthermore reveals theoretical and computational challenges.
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10/11/2009 (14.00) [Lieu : Bât. Euler] Michel
GEVERS (CESAME) "Identifiability, informativity, information matrix and the
Prediction Error criterion: a new look at the connections
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(slides)
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| This seminar will present the results of 3 years of work
with Alex Bazanella (Universidade Federal do Rio Grande do Sul),
Xavier Bombois (Delft University) and Ljubisa Miskovic (EPFL,
Lausanne) on the connections between four different ingredients in
Prediction Error Identification: the information content of the data,
the identifiability of the model structure, the information matrix,
and the minimum (or minima) of the Prediction Error Criterion. We show
that by introducing a new concept of local informativity of the data,
one can establish a number of nice equivalences between these four
ingredients.
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03/11/2009 (14.00) [Lieu : Bât. Euler] Christophe
DE VLEESCHOUWER (TELE, UCL) "Multi-sensored vision for autonomous production of personalized video
summaries"
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Today's media consumption evolves towards increased user-centric adaptation
of contents, to meet the requirements of users having different expectations
in terms of story-telling, and heterogeneous constraints in terms of access
devices. Individuals and organizations want to access dedicated contents
through a personalized service that is able to provide what they are
interested in, at the time when they want it, and through the distribution
channel of their choice. In this talk, we explain how it is possible to
address this challenge by merging computer vision tools with optimization
and resource allocation mechanisms, to automate the collection and
distribution of audiovisual contents.
In a typical application scenario, the sensor network for media acquisition
is composed of (microphones and) cameras, which, for example, cover a
basket-ball field. Distributed analysis and interpretation of the scene are
exploited to decide what to show or not to show about the event, so as to
produce a video composed of a valuable subset from the streams provided by
each camera. The process involves numerous integrated technologies and
methodologies, including but not limited to automatic scene analysis, camera
viewpoint selection and control, and generation of summaries through automatic
organization of stories. Considering the problem in a multi-camera
environment not only mitigates the difficulty of scene understanding caused
by reflection, occlusion and shadow in the single view case, but also offers
higher flexibility in producing visually pleasant video reports. In final,
multi-camera autonomous production/summarization can provide practical
solutions to a wide range of applications, such as personalized access to
local sport events through a web portal or a mobile hand-set, cost-effective
and fully automated production of content dedicated to small-audience, e.g.
souvenirs DVD, university lectures, conference, etc, and interactive
browsing and automated summarization for video surveillance.
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27/10/2009 (14.00) [Lieu : Bât. Euler] John
LEE (Unité d'imagerie moléculaire et radiothérapie expérimentale, UCL) "Dimensionality reduction: from PCA to recent nonlinear techniques" (slides)
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Dimensionality reduction is an old yet unsolved problem, with many applications in data visualization, knowledge discovery, and machine learning in general.
Our aim in this talk will be to review several developments in the field of dimensionality reduction, with a particular focus on nonlinear methods.
As an introduction, we will point out some weird properties of high dimensional spaces, which will motivate the use of dimensionality reduction.
Next, we will go back in time and start our review with a short reminder about well known techniques such as principal component analysis and multidimensional scaling.
Our travel into time will also bring us to visit Sammon mapping and other methods based on distance preservation.
Next, we will come across self-organizing maps and auto-encoders with bottleneck neural networks.
Some spectral methods such as Isomap and locally linear embedding will be reviewed as well.
A glance at recent methods based on similarity preservation such as stochastic neighbor embedding will close the survey.
Finally, we will try to identify the relationships between the different approaches, and say a few words about quality criteria for dimensionality reduction techniques.
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20/10/2009 (14.00) [Lieu : Bât. Euler] Jan
MACIEJOWSKI (University of Cambridge) "Fault-tolerant control - is it possible?"
(slides)
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A real demand now exists for fault-tolerant control systems.
This talk gives some examples in which this arises, drawn from process
control, automotive and aerospace applications. But fault-tolerance is a
very ambitious objective; the paper examines critically what has been
achieved so far, and what it might be possible to achieve. It also
criticises current academic work on fault-tolerant control, and speculates
a little on how progress might be achieved. The paper is illustrated by
examples from real applications, particularly from the aerospace sector.
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13/10/2009 (14.00) [Lieu : Bât. Euler] Richard
BRAATZ (University of Illinois at Urbana-Champaign) "Robust Optimal Control of Finite-time Distributed Parameter Systems"
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Most products of high value such as in the pharmaceuticals, microelectronic, and nanotechnology industries are manufactured in a series of processing steps that operate over finite time.
These processes are usually distributed parameter systems in which tight control is required. Computationally efficient methods are proposed for the robust optimal control of finite-time distributed
parameter systems (DPS), in which robustness is ensured for either deterministic or stochastic parametric uncertainties. In the deterministic case, the effects of uncertainties on the states and product
quality are quantified by power series expansions combined with linear matrix inequality or structured singular value analysis. In the stochastic case, the effects of uncertainties are quantified by power
series or polynomial chaos expansions followed by Monte Carlo simulation. The robust performance analysis have been incorporated into fixed or model predictive control algorithms. The approaches
are illustrated for several applications problems.
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08/10/2009 (16.30) [Lieu : Auditoire Ch.-J. de La Vallée Poussin (cycl 01)
] Michel
WEISS (Aberdeen) Colloquium CESAME-MAPA
"Espaces de surfaces et catégories de surfaces (conjecture de Mumford)"
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Soit M l'espace des surfaces connexes, orientées, de genre g, plongées dans l'espace euclidien de dimension n.
On suppose que g et n sont très grands. L'espace M est très loin d'être simplement connexe.
Toutefois la conjecture de Mumford, dans une formulation contemporaine, est une description de
la meilleure approximation de M par un espace simplement connexe. Selon la conjecture celle-ci s'obtient
à partir de M par un procédé de "scanning". L'étude de certaines catégories de surfaces a beaucoup contribué à la preuve
de la conjecture.
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06/10/2009 (14.00) [Lieu : Bât. Euler] CHANG Chia-Tche
, GORISSEN Bastien (CESAME) "Fast computation on SO(3) of optimal oriented bounding box"
(slides)
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Photorealistic realtime rendering and physically correct simulations of
3D scenes are at the bleeding edge of what can be done with today's
computers. Such applications involve collision detection and visibility
tests using methods based on oriented bounding boxes (OBB) enclosing 3D
objects. Joseph O'Rourke published in 1985 an algorithm that computes the
exact solution in cubic time, but is extremely hard to implement. In
practice, faster PCA-based and brute force heuristics are used. The
computation of the minimal-volume OBB is formulated as an unconstrained
optimization problem on the rotation group SO(3). It is solved using a
hybrid method combining the genetic and Nelder-Mead algorithms. This
method is analyzed then compared to the current state-of-the-art, and it
turns out to be either faster or more accurate.
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29/09/2009 (14.00) [Lieu : Bât. Euler] Pierre
DAYE ((CESAME)) "New model of gaze tracking in 2D: Novel architecture with independent
gaze and head controllers"
(slides)
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Reorienting our gaze recruits both eye and head movements to achieve a
common goal. During a gaze movement, the head may also be voluntarily
moved for some other purpose, e.g., to align the mouth with food.
Currently, two mechanisms are proposed to explain the coordination of
eye and head movements. In the first, a central gaze controller
pre-computes a desired eye and head displacement to execute a gaze
movement. This model can generate independent movements for the head and
gaze but, because of its structure, it can not compensate for
perturbations of the head during the gaze change. In the second, a gaze
controller sends the same command to the eye and the head, and a gaze
feedback loop compensates for any errors or perturbations during the
gaze change. However, this model can not generate independent movements
of the head. We propose a novel approach for coordinated eye and head
control in which there are separate controllers, for gaze and head,
rather than for gaze alone or for eye and head. In this model, the gaze
controller drives both the eye and the head. Gaze feedback through the
cerebellum compensates for perturbations by acting only on the eye.
Thus, in this model the eye only serves gaze, and can not be directed to
an arbitrary goal.
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22/09/2009 (14.00) [Lieu : Bât. Euler] Pierre-Antoine
, Absil (CESAME) "Stiefel Manifolds and their Applications"
(slides)
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Stiefel manifolds, or sets of orthonormal p-frames in R^n, are named after Eduard Stiefel (1909-1978),
who analyzed their topology in the 1930s. This presentation will be a reiteration of a talk that was part of the celebration
of Stiefel's 100th birthday at ETH Zurich. We will count dimensions, describe tangent spaces, visualize embedded and
quotient geodesics, and touch upon applications such as principal component analysis, Lyapunov exponent computation,
and blind source separation.
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15/09/2009 (14.00) [Lieu : Bât. Euler] Brian
ANDERSON (Australian National University) "Multivariable zero-free Transfer Functions and Spectra, and their application in Economic Modelling.".
(slides)
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Central banks and funds investment managers work with mathematical models. In recent years, a new class of model has come into prominencegeneralized
dynamic factor models. These are characterized by having a modest number of inputs, corresponding to key economic variables and industry-sector-wide variables
for central banks and funds managers respectively, and a large number of outputs, economic time series data or individual stock price movements for example.
It is common to postulate that the input variables are linked to the output variables by a finite-dimensional linear time-invariant discrete-time dynamic model, the
outputs of which are corrupted by noise to yield the measured data. The key problems faced by central banks or funds managers are model fitting given the output
data (but not the input data), and using the model for prediction purposes.
These are essentially tasks usually considered by those practicing identification and time series modelling. Nevertheless there is considerable underlying linear system
theory. This flows from the fact that the underlying transfer function matrix is tall.
This presentation will describe a number of consequences of this seemingly trivial fact. For example, a tall transfer function of known McMillan degree but otherwise
generic has no zeros, finite or infinite. A finite sequence of output data in the discrete time case allows recovery of a finite sequence of input data, without knowledge
of the initial state. Canonical state-variable forms take on a special structure, with the number of real parameters growing linearly with the number of outputs, rather
than, as usual, quadratically.
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26/08/2009 (16.30) [Lieu : Aud. Ch.-J. de la Vallée Poussin (Cycl01)] Svetlana
BUTLER (University of Illinois at Urbana-Champaign
) Colloquium CESAME-MAPA "Quasi-linear functionals and topological measures"
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This talk is an introduction to the theory of quasi-linear functionals and topological measures.
Quasi-linear functionals are functionals on C(X) which, rather than being linear on all of C(X), are only
required to be linear on singly generated subalgebras of C(X). The corresponding set-functions on X
are topological measures.
Quasi-linear functionals and topological measures generalize linear functionals and Borel
measures and have many fascinating properties. We will give basic definitions and examples,
discuss some major results in the field so far, and outline venues for further research in this new area of
mathematics.
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30/06/2009 (14.00) [Lieu : Bât. Euler] Raphaël JUNGERS (CESAME
) "The joint spectral radius: theory and applications."
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I will survey recent results on the joint spectral radius, and more generally on asymptotic properties of products of matrices.
I will put emphasis on nice applications and on open problems, like the finiteness property.
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04/06/2009 (16.30) [Lieu : Bât. Euler] Pierre ROUCHON (Ecole des Mines, Paris) Colloquium CESAME-MAPA
"Fonctions Gevrey et contrôle frontière de certaines EDP"
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| Après des rappels historiques et définitions sur l'ordre Gevrey des fonctions indéfiniment dérivables,
nous montrerons leur intérêt pour la planification de trajectoires de certains systèmes décrits par des
équations aux dérivées partielles avec contrôle au bord. Les deux exemples traités seront les suivants:
équation de la chaleur avec une frontière libre (problème de Stefan); une particule quantique dans une boite.
Nous montrerons d'une part, en quoi ces deux exemples sont représentatifs de la platitude différentielle pour
les systèmes de dimension infinie, et d'autre part l'intérêt, encore mal compris, des techniques de re-sommation
sur les développements en séries des trajectoires.
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26/05/2009 (14.00) [Lieu : Bât. Euler] Damien FRANCOIS (CESAME) "Subset selection for prediction analysis of NIR Spectra"
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| Spectrometric data involve very high-dimensional observations issued from sampled spectra. The correlation of the resulting spectral variables and
the large number of them imply several difficulties in modelling. This talk will propose a method for clustering the variables in order to perform
subset selection, in order to reduce dimensionality, colliearity, and therefore complexity. Experiments on real world data show that the reduction
in redundancy and in number of features leads to better performances obtained using a very low number of spectral ranges.
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19/05/2009 (14.00) [Lieu : Bât. Euler] Ilse IPSEN (North Carolina State University ) "Subset Selection"
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| Subset selection methods try identify those columns of a matrix that are
"most" linearly independent. Many subset selection methods are based on a
QR decomposition with column pivoting. We discuss deterministic and
probabilistic methods for subset selection, as well as the application of
subset selection to nonlinear least squares problems in parameter estimation.
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12/05/2009 (14.00) [Lieu : Bât. Euler] Semiha TURKAY (Institute of Automatic Control, Technical University of Munich
) "Influence of tire damping on the ride performance potential of quarter-car active suspensions"
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| In this study, constraints on the transfer functions from the road disturbance to the
vertical acceleration, the suspension travel, and the tire deflection are derived for a quarter-car
active suspension systems using the vertical acceleration and/or the suspension travel
measurements for feedback. The influence of tire damping on the design of an active
suspension system is illustrated by a mixture of the LQG methodology and the interpolation
approach. Besides, H2 and mixed H2/H? syntheses of the quarter-car model excited by random
road disturbances are studied. It is demonstrated that the influence of tire damping on the
closed-loop performance of the active suspension system can be significant. Finally, the result
is extended to suspension models with polytopic tire damping uncertainties
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05/05/2009 (14.00) [Lieu : Bât. Euler] Cristina STOICA (SUPELEC-FR.) "Robustification hors ligne de lois de commande prédictives multivariables"
|
| Cette présentation propose une méthodologie hors ligne pour la robustification de lois de commande prédictives multivariables,
se basant sur une problématique doptimisation convexe dun paramètre de Youla. Le point de départ de la démarche consiste
à synthétiser une loi de commande initiale prédictive multivariable sous forme détat qui stabilise le système. Le but est de garantir
la robustesse en stabilité face à des incertitudes non structurées et dassurer des performances nominales pour le rejet de perturbations,
imposées sous la forme des gabarits temporels sur les sorties. Ce problème doptimisation est résolu par un formalisme LMI. Le paramètre
de Youla obtenu permet de gérer dune part le compromis entre la robustesse en stabilité et les performances nominales et dune autre part
permet de réduire linfluence du couplage multivariable sur le rejet des perturbations.
Le cas de systèmes incertains appartenant à un ensemble donné dincertitudes polytopiques est également traité. Dans ce cas, une condition
supplémentaire BMI est ajoutée pour chaque sommet du polytope considéré, afin de garantir la stabilité sur tout le polytope. Il sagit dun problème
doptimisation non-convexe pour lequel une solution de complexité raisonnable sous une forme LMI sous-optimale est proposée. Loriginalité consiste
à pouvoir garantir la stabilité avec le correcteur robustifié pour le domaine incertain considéré, même dans le cas où le correcteur MPC initial est instable
sur une partie du polytope.
|
28/04/2009 (14.00) [Lieu : Bât. Euler] Iven MAREELS (Dept. of Electrical and Electrotechnic Eng., University of Melbourne) "Water Information Networks: Successes and Challenges"
(slides)
|
It is estimated that we harvest and utilize about 65% of the readily available fresh water resources of the world. In general, perhaps
because water is perceived as an abundantly available resource, we use water rather poorly. Typically less than half the water taken from
the environment serves the objective for which it was intended. The UNESCO World Water reports 2003 and 2005 identify in no uncertain terms a water crisis.
In this lecture we provide an overview of a 10 year collaborative research and development effort, between the University of Melbourne
and a local company Rubicon Systems Australia, and more recently with National ICT Australia.
The programme called Water Information Networks (WIN) is a systems engineering approach to water management in irrigation systems.
Because irrigation accounts for 70% of the total water consumption, this is a logical place to start. The ultimate goal is to manage water at the level of an entire
water catchment basin, accounting for surface and ground water and providing for the needs of all users, including the environment. WIN has developed a sensor/actuator
network and a systems engineering approach to water management. The patented technology (commercialized as Total Channel Control) is now being deployed in Australias
largest irrigation district Goulburn Murray Water (GMW), consisting of 6800km of open irrigation canals servicing over 22,000 farms.
The objective for the open canal system is to deliver water on demand (in as much this may be feasible) with maximal overall efficiency meeting the competing demands.
We review the research work, including open questions, and discuss the WIN outcomes from a number of substantial pilot and commercial projects in Australia that have realized
significant gains in either water efficiency or water productivity in irrigation.
|
23/04/2009 (14.00) [Lieu : Bât. Euler] Robert BITMEAD (University of California, San Diego
) "Communications for Coordinated Control via Covariance Management"
|
|
Interaction between coordinating systems requires communication of
information between the systems. Our aim is to quantify the required level
and content of this communication. In the example considered, the interaction is developed as a non-collision constraint between
vehicles and each vehicle solves a constrained Model Predictive Control (MPC) problem to
determine its path - with the non-collision constraint being central.
The extension of MPC from a full-state feedback strategy to one involving state
estimates allows the adoption of the estimate covariance as a means to manage the communication requirements. This, in turn, admits a
formulation of the communication resource assignment problem as a set of coupled Linear Matrix Inequalities, which link the control objectives, the external
disturbance, and the communications.
|
21/04/2009 (14.00) [Lieu : Bât. Euler] Michel JOURNEE (Ulg) "Optimization methods for sparse principal component analysis"
|
|
Given an m-by-n matrix that stores m samples of n random variables, sparse principal component analysis (sparse PCA)
is the problem of finding a linear combination of these variables that is sparse ( i.e., that involves as few as possible of the original variables)
but that still captures as much as possible of the variance in the data. Sparsity is typically enforced for the sake of interpretability: components
that are linear combinations of a small number of the original variables are easier to interpret. Sparse PCA has applications in virtually all areas
of science, where large data sets need to be analyzed.
This talk is organized in two parts. First, we formulate sparse PCA as the maximization of a convex function on a compact set and propose a simple
gradient-method for this class of problems. We then consider convex relaxations of sparse PCA, which are optimization problems for positive semidefinite
matrices of potentially large dimension. For the sake of numerical efficiency, we discuss an approach that rests on low-rank positive semidefinite matrices,
which entail a quotient manifold structure.
|
31/03/2009 (14.00) [Lieu : Bât. Euler] Marco DORIGO (IRIDIA/ULB) "The Swarm-bot Experience in Swarm Robotics"
|
| A swarm-bot is an artifact composed of a swarm of assembled robots called s-bots.
The s-bots are mobile robots capable of connecting to, and disconnecting from,
other s-bots. S-bots have relatively simple sensors and motors and limited compu-
tational capabilities. In the swarm-bot form, the s-bots are attached to each
other, forming a single robotic system that can move and change its shape.
A swarm-bot can solve problems that cannot be solved by s-bots alone, and is par-
ticularly robust to malfunctioning components.
In the talk, after shortly describing the s-bots hardware and the
methodology we followed to develop algorithms for their control, I will focus on
the capabilities of the swarm-bot robotic system by showing video recordings of
some of the many experiments we performed to study coordinated movement, path
formation, self-assembly, collective transport, shape formation and
reconfiguration, distributed fault detection and other collective behaviors.
|
24/03/2009 (14.00) [Lieu : Bât. Euler] Emile SIMON (CESAME) "LMI formulation for Optimal Control of coal fired Power Plants"
|
|
Thermal power plants are usually controlled by classical regulations
(mostly PIDs), developed with a decentralised model, splitting the system
in many different SISO loops. However, multiple providers and distributed
generation are more likely to cause sudden and large changes of the load
demand. Improving the performance is thus a commercial commodity of great
importance in the current markets state of privatised power industry.
Therefore, we consider the solution of an optimal multivariable control.
The objective pursued here was already studied in a previous work
["Performance Limitations arising in the Control of Power Plants"
presented the 16/12/08 by Vincent Wertz], a key feature was the relation
between the value of the tracking cost and the delay structure of the
plant.
Here a new method is chosen, the problem is cast into an LMI formulation.
At first we develop the same trade-off results as previously found. But
now the new formulation allows one to add constraints with other LMI's.
Since in this paper the control energy is taken into account, a new LMI
reducing the control input is developed.
Without constraint, the coal flow input was too large. So a new LMI is
introduced, allowing to reduce the norm of the optimal controller and thus
the size of the input produced. This is shown to have a limited effect on
the original objective, meaning a slight impact on the performance.
We conclude from the results that the control is significantly improved,
while having control inputs within their limits.
|
10/03/2009 (14.00) [Lieu : Bât. Euler] Daniel COUTINHO (Pontifícia Universidade Católica do Rio Grande do Sul, Brazil.) "An LMI approach for uncertain nonlinear systems".
(slides)
|
| In the last two decades, LMI-based techniques have been successfully applied to cope with uncertain linear
systems in a wide diversity of applications such as robustness analysis, control and filter design, H2 and H-infinity design,
control saturation and state-delayed systems. Together with the evolution of available LMI solvers, in terms of reliability,
computational burden and easily of coding, many researchers have developed different LMI methodologies to deal with
nonlinear systems. Recently, several different LMI-based techniques have been derived for nonlinear systems. One can
divide these works in two main classes: (i) the linear-like representations, and (ii) the SOS (sum of squares) approach.
The linear-like representations preserve the system state space representation and thus they can be applied for synthesis purposes.
However, global stability conditions are of difficult handling. In contrast, the SOS-based approaches may handle global stability
conditions but they are not suitable to control design. In this talk, the Differential-Algebraic Representation (DAR) framework is
introduced to deal with a large class of nonlinear systems. Some recent results on stability analysis, performance and control
synthesis are presented and a discussion is carried out on the advantages and shortcomings of the approach.
|
03/03/2009 (14.00) [Lieu : Bât. Euler] Nicolas GILLIS (CORE) "New variants of nonnegative matrix factorization for sparsity
improvement and maximum biclique finding".
(slides)
|
| Nonnegative Matrix Factorization (NMF) is a data analysis technique which allows
compression and interpretation of nonnegative data. It has been extensively and successfully applied in numerous applications, e.g. text mining, spectral data
analysis, blind source separation, image processing, computational biology, graph clustering, recommendation systems, statistical models, etc. NMF consists in the
factorization of a nonnegative matrix by the product of two low-rank nonnegative matrices. Because of the nonnegativity constraints, NMF is particularly well-suited to achieve decomposition into parts.
In this talk, we first illustrate the usefulness of NMF with some application examples. We then present and analyze several algorithms to solve NMF.
In this study, we introduce two new variants of NMF: Nonnegative Factorization (NF) and Nonnegative Matrix Underapproximation (NMU). NF is a generalization of
NMF while NMU enables a recursive procedure for NMF and is particularly well-suited to achieve a better (sparser) part-based
decomposition. Algorithms and applications for both NF and NMU are presented.Finally, NF and NMU are shown to be NP-hard using a reduction to a graph partitioning problem, namely the maximum-edge biclique problem (MBP).
This reduction allows us design a new type of simple and efficient algorithm for MBP.
|
24/02/2009 (14.00) [Lieu : Bât. Euler] Bruno DEHEZ (ELEC/LEI) "Optimisation topologique : entre algorithmes meta-heuristiques et déterministes".
|
| Les méthodes d'optimisation topologique permettent d'automatiser la conception de toutes sortes de dispositifs (structures mécaniques,
dispositifs électromagnétiques, ...) grâce à l'action d'un algorithme d'optimisation dont l'objectif est de déterminer la meilleure manière de
distribuer la matière dans un espace de design défini. Pratiquement, il sagit de subdiviser cet espace de design en cellules chacune caractérisées
par une série de paramètres fixant les propriétés (perméabilité magnétique, conductivité électrique et densité de courant dans le cas de dispositifs électromagnétiques)
des matériaux les constituant. La recherche de la structure et de ses dimensions optimales est alors réalisée par un algorithme doptimisation qui se charge de
jouer avec les paramètres caractérisant les propriétés du matériau de chacune des cellules.
Voilà maintenant un peu plus d'un an que nous avons initié dans notre laboratoire, en collaboration avec l'ENS de Cachan, des travaux de recherche sur ces
méthodes d'optimisation topologique. Nous suivons deux approches distinctes. La première suppose que les matériaux de chacune des cellules peuvent varier
de manière continue d'un matériau A vers un matériau B, ce qui permet de réaliser des études de sensibilités sur les paramètres, et de ce fait, d'utiliser des algorithmes
d'optimisation déterministes. La seconde approche, par contre, ne considère pas que les matériaux peuvent varier de manière continue. Les algorithmes
d'optimisation utilisés dans ce cas sont des algorithmes méta-heuristique pour lesquels le calcul des gradients n'est pas nécessaire. Bien que la convergence
de ces algorithmes soit a priori plus lente que celle des algorithmes déterministes, ils ont de nombreux avantages, dont celui d'éviter plus facilement le piège de l'optimum local.
Ce séminaire présentera l'état d'avancement de ces recherches ainsi que les premiers résultats obtenus avec les deux approches.
|
16/02/2009 (14.15) [Lieu : Barb.93] Jean-Baptiste COULAUD (CESAME) Thèse de doctorat "Singularités de Saut, Singularités de Platitude et Commande Optimale".
|
| Ce document définit et décrit, dans le cadre des systèmes dynamiques à temps continu, un phénomène de discontinuité entre l'espace des trajectoires
d'état et l'espace des trajectoires des sorties. Ce phénomène, que nous appellerons singularité de saut, apparaît dans certains systèmes dynamiques alors
même que les fonctions des équations différentielles qui en définissent l'évolution sont parfaitement lisses. La thèse analyse le lien entre cette propriété et la
propriété de platitude différentielle. Outre la compréhension structurelle des systèmes concernés que la notion de singularité de saut apporte, la thèse souligne
aussi les implications sur la formulation de certains problèmes de commande optimale.
Plusieurs exemples viennent illustrer cette problématique, notamment celui du robot mobile à plusieurs roues orientables qui a été le point de départ de l'analyse
|
12/02/2009 (16.30) [Lieu : Ch.-J. de La Vallée Poussin (cycl 01)
] Stefaan VAES (K.U.L.) Colloquium CESAME-MAPA
"Algèbres de von Neumann et actions de groupes dénombrables préservant une mesure de probabilité".
|
| Le sujet de cet exposé se trouve au carrefour de l'analyse
fonctionnelle, la théorie ergodique et la théorie des groupes. En
utilisant une construction due à Murray et von Neumann (1943), les
groupes dénombrables et leurs actions ergodiques sur des espaces mesurés
donnent lieu à des algèbres d'opérateurs sur un espace de Hilbert, que
l'on appelle algèbres de von Neumann. Le but est d'expliquer la relation
subtile entre une action de groupe et son algèbre de von Neumann : un
théorème de Connes de 1976 dit que toutes les actions de tous les
groupes moyennables mènent à une seule algèbre de von Neumann, tandis
que les résultats récents de rigidité de Popa fournissent des actions de
groupes telles que l'algèbre de von Neumann associée se souvient
entièrement du groupe et de l'action.
|
10/02/2009 (14.00) [Lieu : Bât. Euler] Olivier
WHITE (School of Psychology, Bangor, UK) "Responsibility Assignment in Redundant Systems".
|
| Most movements are produced by the combined action of multiple actors or effectors. Such movements are highly
redundant: each effector can substitute for the contribution of others. Recent research suggests that the brain distributes the
work across different effectors such that signal-dependent noise and/or effort are minimized. When an error occurs during
movement, however, another important problem arises: due to the redundancy, it is not clear from which effector the error
originated. Which effector should carry the main responsibility for online error correction? For which effector should the motor
command be updated in the next movement? Optimally, one would solve these questions based on separate principles: the effector
that is better at correcting online errors should perform most of the correction, and the effector that is most likely responsible for the error, should adapt.
We investigated how the brain distributes correction and adaptation across multiple effectors in a series of redundant tasks in which multiple effectors
controlled the position of a single cursor. To probe online corrections and adaptation, the cursor was visually rotated by a random angle on each trial.
We measured how much each effector corrected for the visual error by determining the difference between the initial and the overall movement direction.
Participants showed a strong tendency to correct more with the left hand. We then estimated the learning rates for each of the involved effectors by fitting a
state-space model to the initial movement directions. We found a clear correlation between the distribution of the online correction and the distribution of the adaptation.
Thus, rather employing separate principles, our results clearly show the brain solves the responsibility assignment jointly for correction and adaptation.
In other words, the effector that is found guilty of committing the error corrects more and adapts more.
|
16/12/2008 (14.00) [Lieu : Bât. Euler] Vincent WERTZ (CESAME) "Performance Limitations arising in the Control of Power Plants" (slides)
|
| This talk presents results on performance limitations for direct fired coal power plants. A specific feature of this system is the existence of a very large input delay between one of the inputs, namely coal flow, and the two outputs, load and vapour pressure. This problem motivates the main theoretical question addressed in this paper: To examine tracking performance limitations in one process variable when another process variable is constrained. Our main result makes explicit the performance trade-off between the two conflicting objectives, and also links the achievable performance to the delay structure of the plant. These results give insights into the benefits of MIMO control for power plants and into the necessary trade-off between fast tracking of load step changes and the need for minimizing the variations of the vapour pressure around its nominal value. The results provide a benchmark against which practical controller designs for power plants can be assessed.
|
9/12/2008 (14.00) [Lieu : Bât. Euler] Frédéric CREVECOEUR (CESAME) "Trajectory planning of vertical pointing movements: using gravity to move the arm" (slides)
|
| This study aims at better understanding how the central nervous system (CNS) integrates the action of the gravitational torque on the upper limb when we perform a vertical movement such as replacing a cup on a shelf. Particular kinematic properties of vertical pointing movements have provided evidence for an integration of the action of gravity in the motor plan. However, it remains unclear how the CNS achieves this integration and optimal control approaches considering minimum joint torque input failed to explain the kinematic properties observed in normal gravity condition. In this experiment, we asked subjects to perform vertical pointing movements in different gravitational conditions, normal (1g), hyper (1.8g) and micro gravity (~0g). We used a model that considers minimal motoneuron discharge to test the hypothesis that gravity is integrated in an energy-optimal way. The observed behaviors are consistent with the model's predictions although the strategies vary across subjects, movement's direction and gravity condition. The analysis of subjects' strategies permit to better understand the nature of mechanism underlying adaptation of motor control to altered gravity condition.
|
3/12/2008 (16.00) [Lieu : Aud. BARB92, Place Sainte Barbe] Erik DEMAINE (MIT, U.S.A.) CHAIRE FRANQUI - (Teoretical) Computer Science is Everywhere
|
| Theoretical computer science, and the algorithmic way of thinking,
transcends our traditional boundaries. I believe that algorithms are relevant
to every discipline of study, and will give eclectic examples from the arts and
sciences to business and society. The examples span the spectrum from serious
topics like protein folding and decoding Inka khipu to fun topics like juggling
and magic. |
27/11/2008 (16.30) [Lieu : Auditoire Ch.-J. de La Vallée Poussin (cycl 01)
] Frédéric BOURGEOIS (ULB) Colloquium CESAME-MAPA "Un survol de l'homologie de contact"
|
| L'homologie de contact est un invariant pour les structures de contact, dont la définition est basée sur les courbes holomorphes.
Dans cet exposé, on esquissera la construction de l'homologie de contact, puis on illustrera certaines de ses applications et on discutera de certaines de ses propriétés.
|
25/11/2008 (14.00) [Lieu : Bât. Euler] Michel PERRIER (Ecole Polytechnique de Montréal) "Microbial Fuel Cells : Principles and Optimization by Multi Unit Approach" (slides)
|
|
Multi-unit optimization is an extremum seeking method (real-time
steady state optimization of a dynamic system by controlling the
gradient), where the gradient is obtained using finite difference of
the outputs of multiple identical units driven with different
inputs. The major limitation of this approach is that the
convergence is established only when units are identical. This talk
addresses the convergence of the multi-unit scheme with
non-identical units. With a simple quadratic approximation of the
objective function, it is shown that the scheme converges only when
the difference between the inputs is either larger than or in a
direction opposite to that of the difference between their optimum
values. Also, it is shown that the best value for the difference
between the inputs is not zero, but determined by the deviation
between the units.
The multi-unit algorithm is applied to the maximization of power
production in a stack of two continuous flow microbial fuel cells
(MFCs) to maximize power output by manipulating external resistances
of two aircathode membraneless MFCs. The experiment demonstrated fast
convergence toward optimal external resistance and algorithm stability during external
perturbations (e.g. temperature variations). Rate of the algorithm convergence was much faster
than in traditional maximum power point tracking algorithms (MPPT), which are based on
temporal perturbations.
|
18/11/2008 (14.00) [Lieu : Bât. Euler] Alain SARLETTE (ULg) "Coordination on nonlinear spaces" (slides)
|
Recently, numerous authors have studied algorithms for interacting individual agents that lead to a coordinated behavior of the overall swarm. Mostly, the agents are assumed to evolve on a vector space.
This talk extends the theoretical study of coordination to nonlinear but highly symmetric spaces, like the spheres or Lie groups. The focus is on consequences of the geometry and associated symmetries of the setting. It shows that new fundamental problems appear in this context, proposes appropriate definitions and various control methods to circumvent undesired phenomena. Two main "coordination" tasks are considered: (i) achieving a particular configuration of the agents and (ii) performing "coordinated motions".
|
4/11/2008 (14.00) [Lieu : Euler] Cosmin LAZAR (University Reims, France) "Unsupervised methods for multivariate data analysis" (slides)
|
Multivariate data analysis gathers efficient tools for extracting reliable information in order to comprehend the phenomenon in study. Gathering data into groups or classes according to some similarity criteria is an essential step in the analysis. Intrinsic dimension or dimension reduction of multivariate data, the choice of the similarity criterion, cluster validation are problems which still let open questions.
This work tries to make a step further concerning two of the problems mentioned above: the choice of the similarity measure for data clustering and the dimension reduction of multivariate data. The choice of the similarity measure for data clustering is investigated from the concentration phenomenon of metrics point of view. Non Euclidean metrics are tested as alternative to the classical Euclidian distance as similarity measure. We tested if less concentrated metrics are more discriminative for multivariate data clustering. We also proposed indices which take into account the inter-classes distance (e.g. Davies-Bouldin index) in order to find the optimal metric when the classes are supposed to be Gaussian.
Blind Source Separation (BSS) methods are also investigated for dimension reduction of multivariate data. A BSS method based on a geometrical interpretation of the linear mixing model is proposed. BSS methods which take into account application constraints are used for dimension reduction in two different applications of multivariate imaging. These methods allow the extraction of meaningful factors from the whole data set; they also allow reducing the complexity and the computing time of the clustering algorithms which are used further in analysis.
|
28/10/2008 (14.00) [Lieu : Euler] Tzvetan IVANOV (CESAME) "Applications of Real Rational Modules in System Identification" (slides)
|
| We introduces a real rational module framework in the context of Prediction Error Identification using Box-Jenkins model structures. This module framework, which can easily be extended to other model structures, allows us to solve and/or extend a number of problems related to the computation of error norms that arise in system identification. Our main contribution to system identification is an extension of the asymptotic variance formulas for Box-Jenkins models derived by Ninness and Hjalmarsson to asymptotic auto-covariance with respect to frequency. This auto-covariance is in fact the integral kernel reproducing the tangent space of the model manifold at the true (or fixed) system w.r.t. the Fisher-information metric. We show how to exploit the fact that the tangent space forms a rational module in order to quantify this kernel in terms of the poles and zeros of the true system and the input spectrum.
|
21/10/2008 (14.00) [Lieu : Euler ] Jean-Michel CORON (Université Pierre et Marie Curie, Laboratoire Jacques-Louis Lions, France) "Quelques méthodes pour étudier la contrôlabilité des équations aux
dérivées partielles non linéaires." (slides)
|
| Quand on veut regarder la contrôlabilité locale autour d'un point
d'équilibre d'un système de contrôle non linéaire modélisé par une
équation aux dérivées partielles, on commence, comme en dimension finie,
par regarder la contrôlabilité du linéarisé. Mais, déjà, à cause d'un
problème de ``pertes de dérivées'', des difficulté apparaissent souvent
pour déduire de la contrôlabilité du linéarisé la contrôlabilité locale
du non linéaire. De plus si le linéarisé n'est pas contrôlable on ne
peut rien déduire pour la contrôlabilité du système non linéaire. Les
méthodes utilisant les crochets de Lie itérés pour traiter ce dernier
cas en dimension finie marchent mal pour les équations aux dérivées
partielles. On présentera d'autres méthodes utiles pour traiter ce cas
(méthode du retour, déformation quasi-statiques, développement en
série). On donnera des exemples récemment traités par ces méthodes.
|
14/10/2008 (14.00) [Lieu : Euler ] Didier HENRION (LAAS-CNRS, University of Toulouse, France, and Faculty of Electrical Engineering, Czech Technical University in Prague, Czech Republic ) "Plane geometry and convexity of polynomial stability regions" (slides)
|
| The set of controllers stabilizing a linear system is generally non-convex in the parameter space. In the case of two-parameter controller design (e.g. PI control or static output feedback with one input and two outputs), we observe however that quite often for benchmark problem instances, the set of stabilizing controllers seems to be convex. In this note we use elementary techniques from real algebraic geometry (resultants and B´ezoutian matrices) to explain this phenomenon. As a byproduct, we derive a convex linear matrix inequality (LMI) formulation of two-parameter fixed-order controller design problem, when possible.
|
8/10/2008 (11.45) [Lieu : Place Croix du Sud (SUD19)] John N. TSITSIKLIS (MIT, USA) D.H.C. - "Decentralized Detection"
|
| Doctor Honoris Causa - Wednesday, October 8th |
30/09/2008 (14.00) [Lieu : Bât. Euler] Robert DAVID (Cesame) "Modélisation dynamique et commande des procédés d'épuration biologique des eaux usées à boues activées" (slides)
|
| Le procédé d'épuration par boues activées est un traitement biochimique qui dégrade les éléments carbonés et azotés présents dans les eaux usées. Cet exposé met d'abord l'accent sur la modélisation de la décantation des boues, une étape primordiale du procédé, avant de considérer un modèle du procédé entier. La réduction et la linéarisation de celui-ci permet alors une synthèse de commande robuste par retour d'état qui régule le taux d'oxygène et les concentrations en sortie des polluants.
|
23/09/2008 (14.00) [Lieu : Bât. Euler] Huseyin AKCAY (Anadolu University, Turkey) "A Subspace-Based Method for Solving Lagrange--Sylvester Interpolation" (slides)
|
| In this talk, we study the Lagrange--Sylvester interpolation of rational matrix functions which are analytic at infinity,
and propose a new interpolation algorithm based on the recent subspace-based identification methods. The proposed algorithm is numerically
efficient and delivers a minimal interpolant in state-space form. The solvability condition for the subspace-based algorithm is particularly
simple and depends only on the total multiplicity of the interpolation nodes. As an application, we consider subspace-based system identification
with interpolation constraints, which arises, for example, in the identification of continuous-time systems with a given relative degree.
|
16/09/2008 (14.00) [Lieu : Bât. Euler] Brian ANDERSON (Australian National University) "Computational Approaches to H∞ problems and Differential Games" (slides)
|
Linear H∞ problems require the solution of Riccati equations with an indefinite quadratic term, which can be computationally challenging. For H2 Riccati equations, numerical problems can often be overcome using the Kleinman iteration, involving a sequence of Lyapunov equations. While this is not directly possible for Riccati equations with an indefinite quadratic term, an iteration involving a sequence of H2 equations is possible which allows a double iteration involving Lyapunov equations.
These ideas are given a game theory interpretation, which then suggests an extension of the results that will also be exposed; the extension indicates how a Hamilton-Jacobi-Bellman-Isaacs equation for a class of nonlinear game theory problems can be solved by iterating of Hamilton-Jacobi-Bellman equations, each of which in turn can be solved by iteration using linear partial differential equations.
|
30/05/2008 (11.00) [Lieu : Bât. Euler
] Abdellah BENZAOUIA
(EACPI, University Cadi Ayyad,
)
Stability and Control Synthesis of Switching Systems Subject to Actuator Saturation
|
|
This paper presents sufficient conditions for the stabilization of switching discrete-time linear systems subject to actuator saturations.
These conditions are obtained by using successively state and output feedback control laws. The obtained results are formulated in terms of LMI's.
Three sets of LMI's are presented for output feedback case. A numerical example is used to illustrate the technique by using a linear optimization problem subject to LMI constraints.
Key-words: Switching systems, Actuator saturations, invariant sets, Lyapunov functions, LMI's.
|
20/05/2008 (14.00) [Lieu : Bât. Euler
] Pierre Daye
( CESAME
)
Adding head to eye movements: what are the implications for visual tracking?
|
|
In everyday life, humans make combined eye-head movements to reorient their gaze
(Gaze = Eye movement with respect to the head + head movement with respect to the torso).
Combined eye-head movements are necessary when the targets of interest are outside the so-called "oculomotor range".
In this seminar, we will first present the main characteristics of combined eye-head movements. We will then show and
discuss results from a 2D gaze orientation task to memorized targets.
|
24/04/2008 (16.30) [Lieu : Bât. Euler
] Pascal Lambrechts
( UCL
)
La conjecture de Poincaré et la classification des 3-variétés.
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D'après le magazine Science, l'évènement scientifique de l'année 2006 fut la résolution de la conjecture de Poincaré
par Grigori Perelman. Dans cet exposé j'expliquerai cette conjecture et des idées élémentaires de sa preuve.
Mais j'expliquerai aussi que Perelman a en fait démontré beaucoup plus que la conjecture de Poincaré puisqu'il semble avoir achevé un formidable programme de recherche initié
par Thurston à la fin des années 1970. Ce programme proposait une classification des variétés de dimension 3 grâce à l'existence de géométries homogènes sur ces variétés, en analogie avec un phénomène analogue en dimension 2 bien connu selon lequel toute surface compacte orientable admet une géométrie à courbure constante.
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22/04/2008 (14.00) [Lieu : Bât. Euler
] Jérémy Badler
( CESAME
)
Interactions Between Explicit and Implicit Timing
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When stimuli are temporally regular, their prediction depends on an
implicit representation of elapsed time. However, it is unclear
whether the underlying timing mechanism is separable from a
conscious, explicit perception of time. To address the question,
human subjects had to predict the duration of an interval either
explicitly, with a button press, or implicitly, by accurately
tracking with the eyes a suddenly-moving target. Performing the
tasks concurrently yielded a delay in response times for both
modalities compared to single-task trials. This suggests that implicit
and explicit time representations are partially independent, and invoking
them simultaneously increases the attentional load on the brain.
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15/04/2008 (14.00) [Lieu : Bât. Euler
] Cristobald de KERCHOVE
( CESAME
)
Iterative filtering for a dynamical reputation system
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I introduces a novel iterative method that
assigns a reputation to $n+m$ items: n raters and m objects.
Each rater evaluates a subset of objects leading to a nxm
evaluation matrix with a certain sparsity pattern. From this
evaluation matrix we give a nonlinear formula to define the
reputation of raters and objects. I also provide an iterative
algorithm that superlinearly converges to the unique vector of
reputations and this for any evaluation matrix. In contrast to
classical outliers detection, no evaluation is discarded in this
method but each one is taken into account with different weights
for the reputation of the objects. The complexity of one iteration
step is linear in the number of evaluations, making our algorithm
efficient for large data set. Experiments show good robustness of
the reputation of the objects against cheaters and spammers and
good detection properties of cheaters and spammers.
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08/04/2008 (14.00) [Lieu : Bât. Euler
] Gianna Del Corso
( University of Pisa
)
Evaluating scientific products by means of citation-based models: a first analysis and validation
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Some integrated models for ranking scientific publications together
with authors and journals are presented and analyzed. The models rely
on certain adiacency matrices obtained from the relations of citation,
authorship and publication, which concurr to forming a suitable
irreducible stochastic matrix whose Perron vector provides the
ranking. Some perturbation theorems concerning the Perron vector of
nonnegative irreducible matrices are proved. These theoretical
results provide a validation of the consistency and effectiveness
of our models. Several paradigmatic examples are reported together
with some results obtained on a real set of data.
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18/03/2008 [Lieu :
Heeze, The Netherlands
]
27th Benelux Meeting on Systems and Control - NO SEMINAR !!
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11/03/2008 (14.00) [Lieu : Bât. Euler
] Marcelo MOISAN
( INRIA-Sophia Antipolis
)
Near optimal and reverse time interval observers. Application to
uncertain bioreactors
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In this seminar we propose a new interval observer for the estimation
of unmeasured variables of uncertain bioreactors. The observer is based on a class
of bounded error observers, that make use of a loose approximation of the bacterial
kinetics. We first show how to generate guaranteed upper and lower bounds on the unknown
state, provided that known intervals for the initial condition and the uncertainties are available.
An optimality criterion is then introduced, leading to the definition of an optimal observer.
We show that this criterion allows to determine a set of gains which generates the best
estimates. We extend the observer design introducing reverse time interval
observers. We show that running observers in reverse time improves dramatically the
convergence rate, through a discontinuous interval estimation procedure.
The method is applied to the estimation of the total biomass of an industrial wastewater
treatment plant, demonstrating its efficiency.
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04/03/2008 (14.00) [Lieu : Bât. Euler
] Chafik SAMIR
( Université de Sciences et Technologies de Lille USTL/CESAME
)
A framework of calculus on facial surfaces : Analysis representation and comparison
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There is an increasing interest in analyzing shapes of facial surfaces with many applications including biometrics, facial surgery, video communications, and 3D animation. This interest is fuelled by the advent of different types of scanners that can provide high-resolution measurements of both geometry and texture of human facial surfaces.
One general goal in this work is to develop computational tools for analyzing 3D face data. In particular, we are interested in comparing the shapes of facial surfaces. Such a tool can be used to recognize human beings according to their facial shapes, to measure changes in a facial shape due to a surgery, or to study/capture the variations in facial shapes during conversations and expressions of emotions.
Additionally, another subproblem addressed in here is to find an optimal deformation of one surface into another, under some chosen criterion. These deformations can be useful in defining summary statistics of a collection of faces. For example, we define and compute an it average face (Karcher mean) of a set of people, or for one person under different facial expressions.
Efficient tools for understanding and studying variability in facial shapes are of great importance in our biometric-oriented society, our method using
planar shapes is tested on FRGC v1.0 database, and obtained results are very promising.
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26/02/2008 (14.00) [Lieu : Bât. Euler
] Nadia BENIICH
(University of El Jadida/CESAME
)
Régulation de la température du réacteur tubulaire à l'intérieur du réacteur chimique tubulaire.
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On s'intéresse dans cet exposé à la régulation de la température des réacteurs tubulaires exothermiques à dispersion axiale
et piston dont la dynamique est décrite par des équations aux dérivées partielles non linéaires, en utilisant des commandes aux
bords soumises à des contraintes. Nous proposons quelques contrôleurs adaptatifs qui permettent de conduire la température
du réacteur à l'intérieur dun voisinage d'un profil de référence de rayon ? arbitraire et suffisamment petit.
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21/02/2008 (16.00) [Lieu : BARB93
] Laure NINOVE
( CESAME
)
JEUDI !! THESE DE DOCTORAT "Dominant vectors of nonnegative matrices. Application to information extraction in large graphs".
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Objects such as documents, people, words or utilities, that are related in
some way, for instance by citations, friendship, appearance in definitions
or physical connections, may be conveniently represented using graphs or
networks. An increasing number of such relational databases, as for instance
the World Wide Web, digital libraries, social networking web sites or phone
calls logs, are available. Relevant information may be hidden in these
networks. A user may for instance need to get authority web pages on a
particular topic or a list of similar documents from a digital library, or
to determine communities of friends from a social networking site or a phone
calls log. Unfortunately, extracting this information may not be easy.
This thesis is devoted to the study of problems related to information
extraction in large graphs with the help of dominant vectors of nonnegative
matrices. The graph structure is indeed very useful to retrieve information
from a relational database. The correspondence between nonnegative matrices
and graphs makes Perron-Frobenius methods a powerful tool for the analysis
of networks.
In a first part, we analyze the fixed points of a normalized affine
iteration used by a database matching algorithm. Then, we consider questions
related to PageRank, a ranking method of the web pages based on a random
surfer model and used by the well known web search engine Google. In a
second part, we study optimal linkage strategies for a web master who wants
to maximize the average PageRank score of a web site. Finally, the third
part is devoted to the study of a nonlinear variant of PageRank. The simple
model that we propose takes into account the mutual influence between web
ranking and web surfing.
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14/02/2008 (14.00) [Lieu : BARB93
] Julien HENDRICKX
( CESAME
)
JEUDI!! THESE DE DOCTORAT "Graphs and networks for the analysis of autonomous agent systems".
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05/02/2008 (14.00) [Lieu : Bât. Euler
] Paul BELL
(University of Liverpool/CESAME )
Computability in Linear Dynamical Systems
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We shall present some current results of computational problems on
matrix semigroups and low dimensional dynamical systems with an
emphasis on reachability and recurrence type problems. In such
problems we are given a finite set of matrices over a semiring and
wish to decide whether certain properties of the semigroup generated
by this set are algorithmically decidable or not. These problems
very often have interpretations in terms of linear dynamical systems
and we shall highlight the connection between these two subjects. We
shall also see some matrix interpretations of Skolem's problem
concerning linear recurrent sequences. Open problems in these areas
and possibilities for future work will also be presented.
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29/01/2008 (14.00) [Lieu : Bât. Euler
] Maher MOAKHER
( Ecole Nationale d'Ingenieurs de Tunis
)
A Riemannian Framework for the Regularization of Symmetric Positive-Definite Tensor Fields
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In this talk we present a Riemannian framework for the smoothing of data that are constrained to live in the space of symmetric
positive-definite matrices. We start by giving the differential geometry of this space. We then use the harmonic map and minimal
immersion theories to construct three flows that drive a noisy field of symmetric positive-definite data into a smooth one.
The harmonic map flow is the equivalent of the heat flow or isotropic linear diffusion which smooths data everywhere.
A modification of the harmonic flow leads to a Perona-Malik like flow which is a selective smoother that preserves edges.
The minimal immersion flow gives rise to a nonlinear system of coupled diffusion equations with anisotropic diffusivity.
Some preliminary numerical results are presented for synthetic diffusion tensor MRI data.
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08/01/2008 (14.00) [Lieu : Bât. Euler ] Ignasi COS-AGUILERA
(HWNI, UC Berkeley, USA)
An Introduction to Dynamic Encoding
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The Brain Machine Interface paradigm relies on cortical adaptivity to
generate and control movement of an external device. In particular,
previous experiments have demonstrated the validity of motor brain areas
for controlling a cursor on a computer screen with the help of visual
feedback. However, these approaches lack control of every parameter
involved in the motion of a physical object, the parameters controlling
the dynamics of the movement: inertia, stiffness and viscosity.
In a complementary fashion to this main stream, we argue that decoding of those parameters from the cortex is the most natural
procedure to control a prosthetic device. Eventually, dynamic control should be the one requiring the least effort to adapt to.
From this perspective, we have been studying the procedures of control of movement within force-field conditions via psychophysical
experiments, aiming at understanding the adaptation process of the brain to situations requiring accountancy of the dynamics of motion.
The experiments demonstrate different strategies to cope with different force-field conditions, suggesting that once adaptation is complete,
movement is controlled via modulation of impedance. Stiffness, viscosity and intertia are optimized according to the position and type of reach.
We present some of these experiments together with a prospect of future research.
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23/06/2006
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